Generally, high intensity discharge (HID) lamps, such as mercury-arc or sodium vapor lamps for example, have a negative resistance impedance with a maintaining voltage which is a function of arc tube temperature. Thus, a ballast inductor is ordinarily employed to limit the current flow with respect to voltage of the lamp. However, the result is limited power available at the lamp and a relatively long warm-up period before the desired lighting is attained. Moreover, the inductor-type ballast circuitry is relatively inefficient, undesirably heavy and cumbersome, and subject to poor power regulation whenever line voltage fluctuations are encountered.
Attempts to overcome the above-mentioned disadvantages led to the development of electronic ballast circuits such as ringing-choke converters, push-pull inverters, and switching regulators. However, the ringing-choke converter tends to suffer from poor operating efficiency while the push-pull inverter is plagued with relatively poor regulation and an excess of magnetic components. Thus, the switching regulator type of circuit appears most suitable for ballast circuit applications.
Although switching regulator type circuity has been and still is employed in HID lamp apparatus, it has been found that presently known circuitry does leave something to be desired. More specifically, it has been found that the known switching regulator type circuitry for HID lamps is relatively expensive of components and assembly labor costs while leaving much to be desired with respect to efficiency and power consumption.